Process for the polymerization of ethylene and interpolymers thereof

ABSTRACT

A novel continuous gas phase polymerization process for producing polyethylene and interpolymers of ethylene and at least one other olefin is provided wherein chloroform is used in a specified amount such that the activity of the titanium containing Ziegler-Natta catalyst is increased.

FIELD OF INVENTION

[0001] The present invention relates to a continuous gas phasepolymerization process for producing polyethylene and interpolymers ofethylene and at least one other olefin comprising introducing into apolymerization medium ethylene or ethylene and other olefin(s), aZiegler-Natta catalyst comprising a titanium component and a co-catalystcomponent, and chloroform wherein the chloroform is present in a molarratio of chloroform to the titanium component of the Ziegler-Nattacatalyst from 0.4:1 to about 3.5:1.

BACKGROUND OF INVENTION

[0002] The use of halogenated hydrocarbons with titanium containingZiegler-Natta catalysts for the production of polyethylene is disclosedin U.S. Pat. Nos. 5,863,995; 5,990,251; 4,657,998 and 3,354,139. Ingeneral it is disclosed that the halogenated hydrocarbons may reduce therate of ethane formation, control the molecular weight of thepolyethylene, produce polyethylenes with broad molecular weightdistributions, or provide other effects.

[0003] In U.S. Pat. No. 5,990,251 it is disclosed that halogenatedhydrocarbons are used, in a polymerization process for producingpolyethylene utilizing a titanium based Ziegler-Natta catalyst, forincreasing the catalyst activity in the polymerization. It is furtherstated that the amount of halogenated hydrocarbon must be present in amolar ratio of halogenated hydrocarbon to titanium of the Ziegler-Nattacatalyst from 0.001 to 0.15. Furthermore it is disclosed that when themolar ratio of halogenated hydrocarbon to titanium is too high, theactivity of the catalyst is not appreciably modified or is substantiallyreduced in a continuous polymerization process. It is also stated thatwhen the molar ratio is too low, the catalyst activity is notsubstantially modified.

[0004] In U.S. Pat. No. 5,863,995 there is also reference to catalyticactivity in a process for producing polyethylene using a titaniumcontaining Ziegler-Natta catalyst and a halogenated hydrocarbon in aspecified amount. The patent states that the halogenated hydrocarbon ispresent in a molar ratio of halogenated hydrocarbon to the titanium inthe catalyst of 0.01 to 1.8. It is further stated that the specifiedquantity of halogenated hydrocarbon results in no substantial variationof the average activity of the catalyst.

[0005] In U.S. Pat. No. 3,354,139 there is disclosed the use ofhalogenated hydrocarbons with a Ziegler-Natta catalyst to control themolecular weight of polyethylene prepared in a solution or slurrypolymerization process.

[0006] In U.S. Pat. No. 4,657,998 there is disclosed a catalyst systemcomprising titanium containing catalyst component, isoprenylaluminum anda halohydrocarbon for the production of polyethylene having a broadmolecular weight distribution.

SUMMARY OF THE INVENTION

[0007] Applicants have unexpectedly found that in a continuous gas phasepolymerization process for producing polyethylene and interpolymers ofethylene and at least one other olefin comprising introducing into apolymerization medium the ethylene or ethylene and at least one otherolefin, a Ziegler-Natta catalyst comprising a titanium component and aco-catalyst component, and chloroform wherein chloroform is present in amolar ratio of chloroform to the titanium component of the Ziegler-Nattacatalyst from 0.4:1 to about 3.5:1, the activity of the catalyst isincreased as compared with a process carried out in the absence ofchloroform.

DETAILED DESCRIPTION OF THE INVENTION

[0008] Applicants have unexpectedly found that in a continuous gas phasepolymerization process for producing polyethylene and interpolymers ofethylene and at least one other olefin comprising introducing into apolymerization medium the ethylene or ethylene and at least one otherolefin, a Ziegler-Natta catalyst comprising a titanium component and aco-catalyst component, and chloroform wherein chloroform is present in amolar ratio of chloroform to the titanium component of the Ziegler-Nattacatalyst from 0.4:1 to about 3.5:1, the activity of the catalyst isincreased as compared with a process carried out in the absence ofchloroform.

[0009] The continuous gas phase polymerization process for producingethylene and interpolymers of ethylene and at least one other olefin maybe carried out using any suitable continuous gas phase polymerizationprocess. These types of processes and means for operating thepolymerization reactors are well known and completely described in U.S.Pat. Nos. 3,709,853; 4,003,712; 4,011,382; 4,012,573; 4,302,566;4,543,399; 4,882,400; 5,352,749 and 5,541,270. These patents disclosegas phase polymerization processes wherein the polymerization zone iseither mechanically agitated or fluidized by the continuous flow of thegaseous monomer and diluent. The entire contents of these patents areincorporated herein by reference.

[0010] The polymerization process of the present invention is effectedas a continuous gas phase process such as, for example, a gas phasefluid bed process. A fluid bed reactor for use in the process of thepresent invention typically comprises a reaction zone and a so-calledvelocity reduction zone. The reaction zone comprises a bed of growingpolymer particles, formed polymer particles and a minor amount ofcatalyst particles fluidized by the continuous flow of the gaseousmonomer and diluent to remove heat of polymerization through thereaction zone. Optionally, some of the recirculated gases may be cooledand compressed to form liquids that increase the heat removal capacityof the circulating gas stream when readmitted to the reaction zone. Asuitable rate of gas flow may be readily determined by simpleexperiment. Make up of gaseous monomer to the circulating gas stream isat a rate equal to the rate at which particulate polymer product andmonomer associated therewith is withdrawn from the reactor and thecomposition of the gas passing through the reactor is adjusted tomaintain an essentially steady state gaseous composition within thereaction zone. The gas leaving the reaction zone is passed to thevelocity reduction zone where entrained particles are removed. Finerentrained particles and dust may be removed in a cyclone and/or finefilter. The said gas is compressed in a compressor, passed through aheat exchanger wherein the heat of polymerization and the heat ofcompression are removed, and then returned to the reaction zone.

[0011] In more detail, the reactor temperature of the fluid bed processranges from about 30° C. to about 130° C. In general, the reactortemperature is operated at the highest temperature that is feasibletaking into account the sintering temperatures of the polymer productwithin the reactor.

[0012] The process of the present invention is suitable for thepolymerization of ethylene and interpolymers of ethylene with at leastone or more other olefins. The other olefins, for example, may containfrom 3 to 16 carbon atoms. Included herein are homopolymers of ethyleneand interpolymers of ethylene and the other olefin(s). The interpolymersinclude interpolymers of ethylene and at least one olefin(s) wherein theethylene content is at least about 50% by weight of the total monomersinvolved. Exemplary olefins that may be utilized herein are propylene,1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 4-methyl-1-pentene,1-decene, 1-dodecene, 1-hexadecene and the like. Also utilizable hereinare non-conjugated dienes and olefins formed in situ in thepolymerization medium. When olefins are formed in situ in thepolymerization medium, the formation of interpolymers of ethylenecontaining long chain branching may occur.

[0013] The Ziegler-Natta catalysts utilized herein are well known in theindustry. The Ziegler-Natta catalysts in the simplest form are comprisedof a titanium component and a co-catalyst component comprising at leastone organometallic compound. In the process of the present invention,the components of the catalyst can be introduced in any manner known inthe art. For example, the catalyst components can be introduced directlyinto the fluidized bed reactor in the form of a solution, a slurry or adry free flowing powder. The catalyst can also be used in the form of adeactivated catalyst, or in the form of a prepolymer obtained bycontacting the titanium component with one or more olefins in thepresence of a co-catalyst. The Ziegler-Natta catalyst can optionallycontain magnesium and/or chlorine. Such magnesium and chlorinecontaining catalysts may be prepared by any manner known in the art.

[0014] The co-catalyst component of the Ziegler-Natta catalyst used inthe process of the present invention can be any organometallic compound,or mixtures thereof, that can activate the titanium metal component ofthe Ziegler-Natta catalyst in the polymerization of ethylenehomopolymers and interpolymers. In particular, the organometallicco-catalyst compound that is reacted with the titanium componentcontains a metal selected from Groups 1, 2, 11, 12, 13 and/or 14 of thePeriodic Table of the Elements as published in “Chemical and EngineeringNews”, 63(5), 27, 1985. In this format, the groups are numbered 1-18.Exemplary of such metals are lithium, magnesium, copper, zinc, aluminum,silicon and the like, or mixtures thereof.

[0015] Preferred for use herein are the organoaluminum compounds such asthe trialkylaluminum compounds and dialkylaluminum monohalides. Examplesinclude trimethylaluminum, triethylaluminum, trihexylaluminum,dimethylaluminum chloride, and diethylaluminum chloride.

[0016] The titanium component, with or without co-catalyst, may bedeposited on a carrier. In so doing, there may be used as the carrierany catalyst carrier compound known in the art. Exemplary carriers aremagnesium oxides, magnesium oxyhalides and magnesium halides,particularly magnesium chloride. The catalyst, with or without thecarrier, may be supported on a solid porous support, such as silica,alumina and the like.

[0017] The Ziegler-Natta catalyst may contain conventional components inaddition to the titanium component and the organometallic co-catalystcomponent. For example, there may be added any internal or externalelectron donor(s) known in the art, and the like.

[0018] The Ziegler-Natta catalyst may be prepared by any method known inthe art. The catalyst can be in the form of a solution, a slurry or adry free flowing powder. The amount of Ziegler-Natta catalyst used isthat which is sufficient to allow production of the desired amount ofpolymeric material.

[0019] The polymerization reaction is carried out in the presence ofchloroform, present in a molar ratio of chloroform to the titanium ofthe Ziegler-Natta catalyst from 0.4:1 to about 3.5:1. Preferably thechloroform is present in a molar ratio ranging from about 0.5:1 to about3:1, and more preferably from about 1:1 to about 2:1.

[0020] The chloroform may be introduced into the polymerization mediumas such, or diluted in a liquid hydrocarbon such as an alkane forexample, propane, n-butane, isobutane, n-pentane, isopentane, hexane,cyclohexane, heptane, octane and the like.

[0021] In carrying out the polymerization reaction of the presentprocess there may be added other conventional additives generallyutilized in processes for polymerizing olefins.

[0022] Any conventional additive may be added to the polyethylenesobtained by the present invention. Examples of the additives includenucleating agents, heat stabilizers, antioxidants of phenol type, sulfurtype and phosphorus type, lubricants, antistatic agents, dispersants,copper harm inhibitors, neutralizing agents, foaming agents,plasticizers, anti-foaming agents, flame retardants, crosslinkingagents, flowability improvers such as peroxides, ultraviolet lightabsorbers, light stabilizers, weathering stabilizers, weld strengthimprovers, slip agents, anti-blocking agents, antifogging agents, dyes,pigments, natural oils, synthetic oils, waxes, fillers and rubberingredients.

[0023] The polyethylenes of the present invention may be fabricated intofilms by any technique known in the art. For example, films may beproduced by the well known cast film, blown film and extrusion coatingtechniques.

[0024] Further, the polyethylenes may be fabricated into other articlesof manufacture, such as molded articles, by any of the well knowntechniques.

[0025] The invention will be more readily understood by reference to thefollowing examples. There are, of course, many other forms of thisinvention which will become obvious to one skilled in the art, once theinvention has been fully disclosed, and it will accordingly berecognized that these examples are given for the purpose of illustrationonly, and are not to be construed as limiting the scope of thisinvention in any way.

EXAMPLES

[0026] In the following examples the test procedures listed below wereused in evaluating the analytical properties of the polyethylenes and inevaluating the physical properties of the films of the examples.

[0027] a) Density is determined according to ASTM D-4883 from a plaquemade according to ASTM D1928;

[0028] b) Melt Index (MI), I₂, is determined in accord with ASTM D-1238,condition E, measured at 190° C., and reported as decigrams per minute;

[0029] c) High Load Melt Index (HLMI), I₂₁, is measured in accord withASTM D-1238, Condition F, measured at 10.0 times the weight used in themelt index test above;

[0030] d) Melt Flow Ratio (MFR)=I₂₁/I₂ or High Load Melt Index/MeltIndex; and

[0031] e) Residual Titanium Content in the Product. The residualtitanium content in the product is measured by X-Ray FluorescenceSpectroscopy (XRF) using a Philips Sequential X-Ray Spectrometer ModelPW 1480. The samples of the polymer to be evaluated were compressionmolded into a circular shaped plaque approximately 43 mm in diameter soas to fit the sample holder on the spectrometer and 3 to 5 mm inthickness and having a smooth flat surface. The molded test specimenswere then placed in the XRF unit and the x-ray fluorescence arising fromthe titanium in the test specimen was measured. The residual titaniumcontent was then determined based on a calibration curve obtained bymeasurements from polyethylene calibration specimens containing a knownamount of titanium. The residual titanium content is reported as partsper million (ppm) relative to the polymer matrix.

[0032] f) The productivity of the catalyst or prepolymer (Productivity)is the ratio of pounds of polyethylene produced per pound of catalyst orprepolymer added to the reactor.

[0033] g) The activity of the catalyst is expressed as grams ofpolyethylene per millimole titanium per hour of reaction and per 0.1 MPaof ethylene partial pressure [g PE·(mM Ti)⁻¹·h⁻¹·(0.1 MPa)⁻¹].

[0034] The transition metal component of the Ziegler-Natta catalyst usedin Examples 1-4 herein was prepared in accordance with Example 1-a ofEuropean Patent Application EP 0 703 246 A1. The Ziegler-Natta catalystwas used in prepolymer form, and was prepared in accordance with Example1-b of European Patent Application EP 0 703 246 A1. A prepolymercontaining about 35.7 grams of polyethylene per millimole of titanium,with a tri-n-octylaluminum (TnOA) to titanium molar ratio of about 1.0,was thus obtained.

[0035] The continuous polymerization process utilized in Examples 1-4herein was carried out in a fluidized-bed reactor for gas-phasepolymerization, consisting of a vertical cylinder of diameter 0.74meters and height 7 meters and surmounted by a velocity reductionchamber. The reactor was provided in its lower part with a fluidizationgrid and with an external line for recycling gas, which connects the topof the velocity reduction chamber to the lower part of the reactor, at apoint below the fluidization grid. The recycling line was equipped witha compressor for circulating gas and a heat transfer means such as aheat exchanger. In particular the lines for supplying ethylene,1-hexene, hydrogen and nitrogen, which represent the main constituentsof the gaseous reaction mixture passing through the fluidized bed, feedinto the recycling line.

[0036] Above the fluidization grid, the reactor contained a fluidizedbed ranging from about 270 kilograms to 450 kilograms consisting of alow-density polyethylene powder made up of particles with aweight-average diameter of about 0.7 mm. The gaseous reaction mixture,which contained ethylene, 1-hexene, hydrogen, nitrogen and minor amountsof other components, passed through the fluidized bed under a pressureranging from about 290 psig (2.0 Mpa) to about 300 psig (2.1 MPa) withan ascending fluidization speed of about 1.7 feet per second (52 cm persecond).

[0037] In Examples 1-4 a catalyst was introduced intermittently into thereactor, the said catalyst comprising magnesium, chlorine and titaniumand having been converted beforehand to a prepolymer, as describedabove, containing about 35.7 grams of polyethylene per millimole oftitanium and an amount of tri-n-octylaluminum (TnOA) such that the molarratio, Al/Ti, was equal to about 1.0. The rate of introduction of theprepolymer into the reactor was adjusted to achieve the desiredproduction rate. During the polymerization, a solution oftrimethylaluminum (TMA) in n-hexane, at a concentration of about 2weight percent, was introduced continuously into the line for recyclingthe gaseous reaction mixture, at a point situated downstream of the heattransfer means. The feed rate of TMA is expressed as a molar ratio ofTMA to titanium (TMA/Ti), and is defined as the ratio of the TMA feedrate (in moles of TMA per hour) to the prepolymer feed rate (in moles oftitanium per hour). Simultaneously, a solution of tetrahydrofuran (THF)in n-hexane, at a concentration of about 1 weight percent, wasintroduced continuously into the line for recycling the gaseous reactionmixture. The feed rate of THF is expressed as a molar ratio of THF totitanium (THF/Ti), and is defined as the ratio of the THF feed rate (inmoles of THF per hour) to the prepolymer feed rate (in moles of titaniumper hour). Dinitrogen monoxide (N₂O) was added as a gas to the line forrecycling the gaseous reaction mixture. The concentration of N₂O in thegas phase polymerization medium is expressed in units of parts permillion (ppm) by volume.

[0038] In Examples 1-4 a solution of chloroform (CHCl₃) in n-hexane, ata concentration of about 0.5 weight percent, was introduced continuouslyinto the line for recycling the gaseous reaction mixture. The feed rateof CHCl₃ is expressed as a molar ratio of CHCl₃ to titanium (CHCl₃/Ti),and is defined as the ratio of the CHCl₃ feed rate (in moles of CHCl₃per hour) to the prepolymer feed rate (in moles of titanium per hour).The CHCl₃ was added as a solution in n-hexane to the line for recyclingthe gaseous reaction mixture.

Example 1

[0039] The continuous gas phase process conditions are given in Table 1and the resin properties are given in Table 2. The molar ratio TMA/Tiwas 7. The molar ratio CHCl₃/Ti was 0.5. The molar ratio THF/Ti was 0.3.The concentration of dinitrogen monoxide (N₂O) in the polymerizationmedium was 305 ppm by volume. 1-Hexene was used as comonomer. Underthese conditions a polyethylene free from agglomerate was withdrawn fromthe reactor at a rate of 189 lb/h (85.7 kg/h). The productivity of theprepolymer was 220 kilograms of polyethylene per kilogram of prepolymerwhich corresponds to an activity of 165 [g PE·(mM Ti)⁻¹·h⁻¹·(0.1MPa)⁻¹].

[0040] The polyethylene had a density of 0.917 g/cc, a melt indexMI_(2.16), I₂, of 0.9 dg/min and a Melt Flow Ratio, I₂₁/I₂, of 27.

Example 2

[0041] The continuous gas phase process conditions are given in Table 1and the resin properties are given in Table 2. The molar ratio TMA/Tiwas 7. The molar ratio CHCl₃/Ti was 1.5. The molar ratio THF/Ti was 0.3.The concentration of dinitrogen monoxide (N₂O) in the polymerizationmedium was 332 ppm by volume. 1-Hexene was used as comonomer. Underthese conditions a polyethylene free from agglomerate was withdrawn fromthe reactor at a rate of 215 lb/h (97.5 kg/h). The productivity of theprepolymer was 242 kilograms of polyethylene per kilogram of prepolymerwhich corresponds to an activity of 205 [g PE·(mM Ti)⁻¹·h⁻¹·(0.1MPa)⁻¹].

[0042] The polyethylene had a density of 0.917 g/cc, a melt indexMI_(2.16), I₂, of 0.9 dg/min and a Melt Flow Ratio, I₂₁/I₂, of 27.

Example 3

[0043] The continuous gas phase process conditions are given in Table 1and the resin properties are given in Table 2. The molar ratio TMA/Tiwas 7. The molar ratio CHCl₃/Ti was 2.0. The molar ratio THF/Ti was 0.3.The concentration of dinitrogen monoxide (N₂O) in the polymerizationmedium was 315 ppm by volume. 1-Hexene was used as comonomer. Underthese conditions a polyethylene free from agglomerate was withdrawn fromthe reactor at a rate of 218 lb/h (98.9 kg/h). The productivity of theprepolymer was 269 kilograms of polyethylene per kilogram of prepolymerwhich corresponds to an activity of 240 [g PE·(mM Ti)⁻¹·h⁻¹·(0.1MPa)⁻¹].

[0044] The polyethylene had a density of 0.917 g/cc, a melt indexMI_(2.16), I₂, of 0.8 dg/min and a Melt Flow Ratio, I₂₁/I₂, of 27. TABLE1 Reactor Conditions for Examples 1 through 3 Example 1 2 3 ReactorPressure, psig (MPa) 299 293 293 (2.06) (2.02) (2.02) ReactorTemperature, ° C. 86 86 86 Fluidized Bulk Density, 15.8 16.3 16.1 lb/ft³(g/cm³) (0.253) (0.261) (0.258) Reactor Bed Height, ft (meter) 11.5 11.311.0 (3.51) (3.44) (3.35) Ethylene, mole % 50.0 50.6 50.6 H₂/C₂ ¹ 0.0930.093 0.092 C₆/C₂ ² 0.118 0.112 0.112 TMA/Ti³ 7 7 7 CHCl₃/Ti⁴ 0.5 1.52.0 THF/Ti⁵ 0.3 0.3 0.3 N₂O, ppm by volume 305 332 315 Prepolymer Rate,lb/h (kg/h) 0.86 0.89 0.81 (0.39) (0.40) (0.37) Production Rate, lb/h(kg/h) 189 215 218 (85.7) (97.5) (98.9) Space Time Yield, kg/h-m³ 57.566.6 69.2 Productivity, mass ratio 220 242 269 Activity⁶ 165 205 240Residual Titanium, ppm 6.1 5.5 5.0

[0045] TABLE 2 Resin Properties for LLDPE prepared in Examples 1 through3 Example 1 2 3 Density (g/cc) 0.917 0.917 0.917 Melt Index, I₂ (dg/min)0.9 0.9 0.8 Melt Flow Ratio (I₂₁/I₂) 26.6 26.6 26.6

Example 4

[0046] The process of Example 3 was followed with the followingexceptions:

[0047] 1. the ethylene concentration in the reactor loop was maintainedat 50.0 mole %,

[0048] 2. the molar ratio of hydrogen to ethylene was set to 0.130,

[0049] 3. the molar ratio of 1-hexene to ethylene was set to 0.110,

[0050] 4. the prepolymer addition rate was fixed at 0.80 pounds per hour(0.36 kg/h),

[0051] 5. the TMA to titanium molar ratio was set to 4,

[0052] 6. the dinitrogen monoxide (N₂O) in the polymerization medium wasmaintained at 300 ppm by volume, and

[0053] 7. the molar ratio of chloroform to titanium was varied.

[0054] The molar ratio of chloroform to titanium as shown in Runs A, B,C and D of Table 3 was present in amounts of 2.0:1; 3.0:1; 3.5:1; and0:1. At each of the four molar ratios of chloroform to titanium theactivity of the catalyst was determined and reported. TABLE 3 RunCHCl₃/Ti¹ Activity² A 2:1 250 B 3:1 160 C 3.5:1   90 D 0 75(comparative)

[0055] Under each of these conditions a polyethylene free fromagglomerate was withdrawn from the reactor.

[0056] From the above data in Examples 1-4 and Tables 1, 2 and 3 thefollowing observations may be made. The addition of chloroform (CHCl₃)in a molar ratio of 0.5:1 to 3.5:1 provides an increase in catalystactivity as compared with a process carried out in the absence ofchloroform.

[0057] It should be clearly understood that the forms of the inventionherein described are illustrative only and are not intended to limit thescope of the invention. The present invention includes all modificationsfalling within the scope of the following claims.

We claim:
 1. A process for increasing catalyst activity in a continuousgas phase process for polymerizing ethylene or ethylene and at least oneor more other olefin(s) comprising contacting, under polymerizationconditions, the ethylene or ethylene and at least one or more otherolefin(s) with a Ziegler-Natta catalyst comprising a titanium componentand a co-catalyst component, and chloroform wherein chloroform ispresent in a molar ratio of chloroform to the titanium component of theZiegler-Natta catalyst from 0.4:1 to about 3.5:1.
 2. The processaccording to claim 1 wherein the molar ratio of chloroform to thetitanium component of the Ziegler-Natta catalyst is from about 0.5:1 toabout 3:1.
 3. The process according to claim 1 wherein the molar ratioof chloroform to the titanium component of the Ziegler-Natta catalyst isfrom about 1:1 to about 2:1.